Two distinct halo populations in the solar neighborhood

III. Evidence from stellar ages and orbital parameters ^⋆,⋆⋆

¹ Observatorio Astronómico Nacional, Universidad Nacional Autónoma de México, Apartado Postal 877, CP 22800 Ensenada BC México
e-mail: schuster@astrosen.unam.mx
² Instituto de Astronomía, Universidad Nacional Autónoma de México, AP 70-264, 04510 México, DF, México
e-mail: edmundo@astroscu.unam.mx; barbara@astroscu.unam.mx
³ Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark
e-mail: pen@phys.au.dk

Received: 7 September 2011
Accepted: 2 November 2011

Abstract

Context. In Papers I and II of this series, we have found clear indications of the existence of two distinct populations of stars in the solar neighborhood belonging to the metal-rich end of the halo metallicity distribution function. Based on high-resolution, high S/N spectra, it is possible to distinguish between “high-alpha” and “low-alpha” components using the [α/Fe] versus [Fe/H] diagram.

Aims. Precise relative ages and orbital parameters are determined for 67 halo and 16 thick-disk stars having metallicities in the range −1.4 <  [Fe/H] < −0.4 to better understand the context of the two halo populations in the formation and evolution of the Galaxy.

Methods. Ages are derived by comparing the positions of stars in the log T_eff–log g diagram with isochrones from the Y² models interpolated to the exact [Fe/H] and [α/Fe] values of each star. The stellar parameters have been adopted from the preceding spectroscopic analyses, but possible systematic errors in T_eff and log g are considered and corrected. With space velocities from Paper I as initial conditions, orbital integrations have been carried out using a detailed, observationally constrained Milky Way model including a bar and spiral arms.

Results. The “high-alpha” halo stars have ages 2–3 Gyr larger than the “low-alpha” ones, with some probability that the thick-disk stars have ages intermediate between these two halo components. The orbital parameters show very distinct differences between the “high-alpha” and “low-alpha” halo stars. The “low-alpha” ones have r_max’s to 30–40 kpc, z_max’s to  ≈18 kpc, and e_max’s clumped at values greater than 0.85, while the “high-alpha” ones, r_max’s to about 16 kpc, z_max’s to 6–8 kpc, and e_max values more or less uniformly distributed over 0.4–1.0.

Conclusions. A dual in situ-plus-accretion formation scenario best explains the existence and characteristics of these two metal-rich halo populations, but one remaining defect is that this model is not consistent regarding the r_max’s obtained for the in situ “high-alpha” component; the predicted values are too small. It appears that ω Cen may have contributed in a significant way to the existence of the “low-alpha” component; recent models, including dynamical friction and tidal stripping, have produced results consistent with the present mass and orbital characteristics of ω Cen, while at the same time including extremes in the orbital parameters as great as those of the “low-alpha” component.